1. Field of the Invention
The present invention relates to a method for fabricating silica glass. In particular, the present invention relates to a method for fabricating silica glass using a sol-gel process.
2. Description of the Related Art
With regard to advances in telecommunication systems, optical communication systems have drawn a lot of attention in the relevant industries because these systems successfully perform at very high transmission speeds with little loss of optical signals. Such optical communication systems typically use an optical fiber fetched from a perform made of silica glass as a transmission media.
In general, the silica glass is fabricated by one of a natural quartz process, synthetic quartz process, or sol-gel process. More details on the sol-gel process can be found in U.S. Pat. No. 5,240,488, to Edwin A. Chandross et al., entitled Manufacture of Vitreous Silica Product Via A Sol-Gel Process Using A Polymer Additive, the contents of which are hereby incorporated by reference as background material.
FIG. 1 provides a flowchart providing an overview of a method for fabricating silica glass using a sol-gel process according to the prior art. The fabrication method includes a sol forming step 110, a sol filling step 120, a gel drying step 130, a low heat treatment step 140, and a sintering step 150.
The sol forming step 110 involves mixing a starting material, deionized water, and an additive to form a sol. As the starting material, a fumed silica or silicone alkoxide can be used. As for the additive, any of a dispersion agent, a catalyst or a binder can be used.
The sol filling step 120 involves filling a circular mold with the sol produced by the forming step 110. Here, the circular mold has a cylindrical shape, and a separable rod is arranged at the center of the circular mold. That is to say, the sol fills inside of the circular mold except for the rod. Later, the sol is gelatinized inside of the circular mold.
The gel drying step 130 involves drying the gel after separating the gel from the circular mold. The gel drying step 130 is performed in a temperature & humidity chamber that maintains a constant temperature and relative humidity.
In the low heat treatment step 140, the dried gel is placed in a low-heating device, and goes through a heat treatment at a temperature of 900° C. as the presence of chloride gas is injected to the inside of the low heat device. Afterwards, the remaining moisture inside of the gel and other organic matters like the binder are decomposed, and any metallic impurities and hydroxyl radicals (OH) in the gel are removed.
The sintering step 150 involves performing a vitrification process on the gel by applying heat to the gel subsequent to performing the low heat treatment step 140. The sintering step 150 is performed by heating the dry gel at a temperature over 1300° C. and moving the gel up and down in the sintering furnace under the helium (He) gas or vacuum atmosphere.
FIG. 2 graphically illustrates scattering characteristics of the silica glass according to the prior art. The graph was obtained by mounting a sample of the silica glass on a spectrometer, and light was made incident onto one side of the sample, and a spectrum analysis was made of the transmitted light. In the graph, the scattering distance indicates a relative measurement position, and the scattering intensity indicates the intensity of the scattered light at a corresponding measurement position. As depicted in FIG. 2, a large number of points appear with a maximum intensity being dependent on the measurement position. Each maximum intensity point exhibits a maximum scattering intensity at the corresponding measurement position. Such scattering phenomenon is caused by micro bubbles existing inside of the silica glass. As previously discussed with reference to FIG. 1, the low heat treatment step was originally carried out on the gel in a chlorine gas atmosphere in order to decompose the remaining moisture and organic matters, such as the binder inside of the gel, and to remove any metallic impurities and hydroxyl radicals (OH). Unfortunately, the chlorine gas is not completely removed, and thus remains inside of the silica glass, causing the micro bubbles to be formed therein.
FIG. 3 is a view illustrating part of a fault of the silica glass fabricated in accordance with the related art. As shown in the figure, the white spots are the micro bubbles formed in the silica glass. Primarily, the micro bubbles are the reason that the optical signals that are supposed to progress inside of the silica glass are scattered, and the cracking and micro-bending of the silica glass occur due to the changes in the ambient temperature.
As described above, the major problem of the traditional method for fabricating silica glass using a sol-gel process is that micro bubbles are produced inside of the silica glass because of the remaining chlorine gas after the low heat treatment step.